How important is size of the inside bore of the cartridge tube in mercurial blood pressure instruments?
It is Critical. When the inside bore is too narrow two problems can result. Mercury lag and mercury separation. Mercury lag is the phenomenon that occurs when the air deflates from the cuff faster than the mercury column can respond. When the forces of friction (of the mercury against the inner walls of the cartridge tube) that prevent the mercury from dropping exceed the gravitational forces causing the mercury to drop you get mercury lag. That same friction can actually cause the column of liquid mercury to split – forcing an air pocket that will distort readings. This is known as mercury separation. ANSI/AAMI SP10 requires that the inside bore of the cartridge tube be a minimum of 3.9mm in diameter. Believe it or not, the inside bore on the cartridge tube of many low cost imports is just 3.2mm – well below government standards. On the other hand, most ADC mercury instruments feature a cartridge tube with an inside bore of 5mm – more than 25% larger than government requirements. (The
